The overall goal of our work with Rous sarcoma virus is to contribute towards a description of the molecular biology of replication of this virus. Towards this goal, we have isolated conditional and nonconditional mutations in each viral replication gene, and have begun or intend to apply these mutants to study initiation of infection leading to an integrated provirus, maintenance of the integrated provirus, and the synthesis and assembly of viral proteins and nucleic acids. Recently, we have also begun to work with a virus of domestic Pekin ducks which belongs to the same family as human hepatitis B virus. We are attempting to develop this virus as a model to characterize the replication of this family of viruses.